Chapter 9 The Muscular System

Skeletal Muscle Structure Tendon – connect muscle to bone Fascia – outermost covering; covers entire muscle & continuous w/tendon; separates muscle from adjacent muscles

Skeletal Muscle Structure Coverings: Epimysium – covers entire muscle (under fascia) Perimysium – covers muscle bundle (fascicle) Endomysium – covers each fiber (cell) Sarcolemma – cell membrane

Skeletal Muscle Structure – Cont. Sarcoplasmic reticulum (SR) channels for transport Myofibrils – threads that compose muscle fibers; contain protein filaments: 1. actin – thin 2. myosin – thick Skeletal Muscle Structure

Muscle Fiber (muscle cell) Cisternae of SR – enlarged portions Transverse tubules (T-tubules) – important in muscle contraction Sarcoplasm – cytoplasm

Breakdown of Skeletal Muscle

Parts of a Sarcomere (functional unit of a muscle)

Parts of a Sarcomere Z lines – end points M line – middle I band – on either side of Z line; actin filaments only H zone – on either side of M line; myosin filaments only A band – overlapping actin & myosin filaments

Parts of a Sarcomere

Neuromuscular Junction – junction b/t motor neuron & muscle Motor neuron – carries impulse from brain or spinal cord to muscle Motor end plate – end of muscle fiber; many nuclei & mitochon- dria located here

Neuromuscular Junction Neurotransmitters (ntm) chemicals that help carry impulses Motor unit – 1 motor neuron & fibers that it stimulates Synaptic vesicles – store neurotransmitter; most common – acetylcholine (ACh)

Electron Micrograph Neuromuscular Junction

4 Proteins in Muscle Cells:

Troponin & Tropomyosin 4 proteins are found in muscle cells: actin, myosin, troponin & tropomyosin troponin – appear as globules; provide a binding site for Ca+² tropomyosin – appear as ribbons; cover the myosin cross-bridge binding sites in a relaxed muscle

Sliding Filament Theory (How Muscles Contract) Muscle fiber stimulated by release of ACh from synaptic vesicles of neuron ACh causes impulse to travel to muscle cell membrane Transverse tubules (T-tubules) carry impulse deep into muscle fibers Sarcoplasmic reticulum releases Ca ions (Ca²+) Ca²+ bind to troponin, tropomyosin moves, exposing binding sites on actin filaments

Sliding Filament Theory (How Muscles Contract ) Linkages form b/t actin & myosin Actin filaments move inward, shortening the sarcomere Muscle fiber relaxes when Ca²+ are transported back to S.R. The enzyme cholinesterase (or AChesterase) decomposes ACh

Sliding Filament Theory Relaxed muscle – binding sites on actin are covered by tropomyosin

Sliding Filament Theory Ca²+ binds to troponin Tropomyosin slides out of the way Myosin binds to actin & pulls inward Sarcomeres shorten & muscle contracts

Sliding Filament Theory

Energy for Muscle Contraction ATP (adenosine triphosphate) provides the energy for muscle contraction When ATP is converted to ADP (adenosine diphosphate) by losing the last phosphate, energy is released.

Energy for Muscle Contraction Cells depend on cellular respiration of glucose to synthesize ATP An additional source is creatine phosphate

Energy for Muscle Contraction Creatine phosphate stores excess energy Can be used to convert ADP back into ATP Anaerobic respiration (in the absence of O 2 ) provides few ATP’s, while aerobic resp. (in the presence of O 2 ) provides many ATP’s

Creatine Phosphate High amts. of ATP - ATP is used to Low amts. of ATP – CP is used synthesize CP, which stores energy to resynthesize ATP. for later use.

Importance of Myoglobin l.a. carried by blood to liver; liver can convert l.a. to glucose, but requires ATP (ATP being used for muscle contraction) myoglobin – stores O 2 in muscle cells; gives muscle its red color

Aerobic vs. Anaerobic Respiration

Carried by blood to liver; liver can convert l.a. to glucose, but requires ATP (ATP being used for muscle contraction) Imp. b/c blood supply during muscle contr. may decrease As l.a. accumulates, O 2 debt occurs

Strenuous exercise leads to O 2 deficiency & lactic acid buildup ATP provides energy for muscle contraction Amt. of O 2 needed to convert accumulated l.a. to glucose & restore ATP levels = O 2 debt L.A. accumulation leads to muscle fatigue b/c pH of muscle cell is lowered & muscle cannot contract Oxygen Debt

Muscle cramp – fatigued muscle has lack of ATP needed to move Ca+² back into S.R.; cross bridges not broken Rigor mortis – takes up to 72 hrs. to occur; sarcolemma becomes more permeable to Ca+² & ATP levels insufficient Muscle Cramp

Myogram Pattern or graph of a muscle contraction A single contraction is called a muscle twitch 3 parts: Latent (lag) phase – brief pd. of delay b/t when the stimulus is applied & actual contraction occurs Contraction Relaxation – return to original state

Patterns of Contraction a) Muscle Twitch – single contraction b) Staircase Effect many stimuli closely spaced w/complete relaxation in b/t; each contraction generate incr. force

Patterns of Contraction c) Summation – when the 2 nd stimulus occurs during the relaxation pd. of 1 st contr.; the 2 nd contr. generates more force d) Tetany – when twitches fuse into 1 sustained contr.

Muscle Facts If a muscle is stimulated twice in quick succession, it may not respond the 2 nd time – called refractory period Threshold – the minimum stimulus needed to cause a contraction All-or-none – increasing the strength of the stimulation does NOT incr. the degree of contraction (a muscle contracts completely or not at all)

More Facts Incr. stimulation from motor neurons causes a greater # of motor units to contract & vice versa Called recruitment of motor units Incr. the rate of stimulation also incr. the degree of contraction Muscle tone – a sustained contraction caused by nerve impulses from s.c. to a small # of muscle fibers in the back, neck, etc.; maintains posture

Origin & Insertion Origin – end of muscle that attaches to stationary bone Insertion – end of muscle that attaches to moving bone During contr., insertion is pulled toward origin

Muscle Functions in Groups Prime mover – responsible for most of the movement (ex.- biceps) Synergist – aids the prime mover Antagonist – resists the prime mover & causes movement in the opposite direction (ex. - triceps)

Structural Differences of 3 Types of Muscle Skeletal MuscleSmooth MuscleCardiac Muscle Cells elongated w/multiple nuclei/cell Cells spindle- shaped w/1 nucleus/cell Cells branching w/1 nucleus/cell T-tubules presentNo T-tubulesT-tubules lg.; releases lg. amts. of Ca++; can contract longer (Ca channel blockers) Striated/voluntaryNon-striated/invol.Striated/invol.

Functional Differences of 3 Types of Muscle Skeletal MuscleSmooth MuscleCardiac Muscle Needs nerve impulse for contraction Displays rhythmicity & cells stimulates each other (as in peristalsis) Displays rhythmicity & self-excitation Ca+² binds to troponinCa+² binds to calmodulin Ca+² binds to troponin Not affected by hormones Hormones may affect contraction Hormones may affect rate of contr. Contracts & relaxes rapidly Slower to contract but can maintain contraction longer Contracts & relaxes at a certain rate

Functional Differences - Continued Skeletal MuscleSmooth MuscleCardiac Muscle Not affected by stretching Stretching of fibers may stimulate contr. (ex.-stomach) Remains in a refractory pd. until contraction ends (tetany won’t occur)

Fast Twitch vs. Slow Twitch Muscle Fast TwitchSlow Twitch Contracts quickly, tires easily (sprinter) Contracts slowly, tires slowly (long distance) Fewer mitochondriaMore mitochondria Less myoglobinMore myoglobin White muscleRed muscle Composes smaller muscles (eyes, hands, etc.) Composes lg. muscles (legs, back, etc.)

Levers Parts of a lever: wt., force, pivot 3 types of levers: 1 st class – W-P-F (seesaw/scissors) 2 nd class – P-W-F (wheelbarrow) 3 rd class – W-F-P (forceps)

Bones & Muscles as Levers Forearm bends – 3 rd class lever (biceps attaches at a pt. on the radius below the elbow joint) Forearm straightens - 1 st class lever ((triceps attaches at a pt. on the ulna above the elbow joint)

Bones & Muscles as Levers Standing on tip-toe – 2 nd class lever (P-W-F)